When to Specify a Exhaust / Return Fans in AHU

[cwigg66]

Here's the siuation -

9000 CFM RTU w/ 2.5" ESP
Ducted Return System
Min O.A. = 4000 CFM to make up for constant exhaust requirements.
Unit supplies air to VAV system for a two story wing of a hospital with isolation rooms, etc.
Wing is connected to other portions of the hospital which are conditioned by separate larger air handlers. The doors between the spaces are often held open, so I am designing the wing to be at neutral pressure so as not to affect neighboring spaces.

I have spec'd a Trane unit with a powered exhaust fan, and spec'd the exhaust fan VFD to be controlled via air flow sensors in the O.A. damper and at the exhaust fan itself. The logic will have an exhaust offset equal to the minimum O.A. and will be set to exhaust the actual O.A. minus the minimum O.A., thereby always maintaining a net zero airflow balance in the building.

My questions are these: 1) Is this control sequence viable?
2) At what point (size of system, type of system) do I need to add a return fan to the system?

Thanks for all your help!

Chris

 

[mauricestoker]

If you are going to include an economizer on the RTU, I don't think this setup would work.

Not sure why a VFD would be needed on a constant volume exhaust, unless filter loading is a concern. Supplies typically (especially in hospital environments) have higher filter loading-MERV 14 final filter is what I typically see.

I would be concerned about isolation rooms, as these would be preferably on a separate HEPA'd exhaust. If on a separate exhaust, you may want to think about a motor operated bubble tight damper on the supply duct from the RTU to close on loss of dP across the isolation barrier or loss of control amperage to the exhaust. You don't want an occupied isolation room blowing positive (unless you are talking about burn unit or immuno-suppressed).

I'd recommend having an ME doing the design, and knowing what is in each area.

 

[cwigg66]

Hehe. Sorry for the confusion. First off, is there any way to change the field in my handle to Mech. Civil/Enviro is from a past life, and I can't figure out how to change it. I assure you I am an ME, and do alot of HVAC design work. This problem is just a little beyond my normal scope.

To be more specific, I have 4000 CFM of air being exhausted from dedicated rooftop fans separate from the RTU. So 9000 CFM supply, 5000 max available for return.

The RTU specified has a powered exhaust economizer, with the powered exhaust to be controlled as described above to be able to account for the need to exhaust more air through the RTU when in economizer mode.

The isolation rooms are supplied via single duct terminal boxes set at fixed volume. Each isolation room has a dedicated rooftop mounted exhaust fan which is dampered to allow the T & B company to achieve the required negative pressure. These room are completely fixed at the request of the hospital (i.e. no variation in flow rates).

Sorr for the confusion on this. I am basically just trying to determine when folks decide a separate return fan is needed in addition to the supply fan. It seems my design will do the job, but I want to be sure.

Thanks

Chris

 

[GMcD]

The decision as to whether a return air fan is needed is based on the return air path resistance back to the main AHU. If any amount of ducted return air is used, then a return fan should be used. If this is an acute care medical facility, I'm guessing that there might be some need to have some ducted return air with local zone return air balancing dampers for room to room and zone to zone air pressure control relationships? Then you'll likely need to use a return air fan to insure adequate airflow balance and zone air pressure control.

 

[cwigg66]

Let me add a little to this problem. I have a single rooftop unit serving a two story building with fully ducted returns. If I use return fans, how do I control them? Should they both be driven simultaneously with the supply fan? The building is VAV controlled, so each floor will have different demands. Furthermore, the building is connected to another building, so how do I ensure that I don't transfer air to and from the adjacent building? This seems like the worst situation possible as far as controls go, but it is a healthcare facility so somewhat more elaborate controls with more hardware is expected to ensure quality. Thanks for your input thus far guys. Very helpful.

Chris

Chris W., P.E.
Mechanical Engineer
Arco Engineering, Inc.

 

[cdxx139]

With a VAV the SF will ramp up and down to maintain the duct static setpoint. I am assuming VAV boxes.

If the return ductwork also have VAV boxes, then the RF can also be measured on static. If you do not have return VAV boxes then...

The RF can track the supply fan, minus any pressurization and/or exhaust. This will require Air Flow Measuring Stations (which I think is a good idea anyways)

knowledge is power

 

[317069]

Hello and Happy New Year all.

As I understood:
- The total cfm is 9000 cfm
- Maximum Return is 5000 cfm
- Minimum Outside Air is 4000 cfm
- Constant separate exhaust rate at 4000 cfm
- Unit selected: Trane roof top unit with economizer
your system could be fine since your power exhaust fan will exhaust less than 40% of the total supply air which is 9000 cfm(your Max return is 5000cfm which is already about 55% of the total air),
But your power fan is on/off type, so you can NOT control it based on the rate of the OA, I mean it doese NOT change the exhaust rate if the OA rate has been changed.
in General, your project is a part of the building and you said that there will be alwayes open doors between your project and the rest of the building, So I don't think you have to worry too much about balancing, but if you want to do something, You could set the power fan exhasut at acertain rate and provide a separate exhaust system controled by the OA sensor or use barometric relife dampers in each story.

for the second question(return fan)
you better to check with the unit manufucturer but in general I can say:
- your total ESP is 2.5"
you have to know how much pressure drop do you have in the supply and return ducts, and check with the RTU fan capacity then you can decide whether you need return fan or not.

 

[KiwiMace]

You need to use a common return fan tracking the SAF.

 

[cwigg66]

Here's where I am with this now -

The plan is to incorporate a single return fan into the RTU, and place dampers at the main return ducts on each floor, to be controlled using static pressure sensors on each floor. The pressure sensors will allow the dampers to be adjusted to maintain neutral pressure on each floor regardless of supply conditions.

Now I just have to figure out the logic behind the controls.

Thanks to everyone for your input with this!

Chris

Chris W., P.E.
Mechanical Engineer
Arco Engineering, Inc.

 

[Drazen]

Supply air volume is apparently function of outside temperature.

As control relation is proprtional, e.i. linear, can't you apply the same relation-cotrol logic to return fan controller? Static pressure just complicates things.

 

[ChasBean1]

Hi cwigg66,

I like your last post better than the original idea of a packaged unit trying to maintain balance in this system by modulating a power exhaust fan. These types of systems have a one-size-fits-all mentality whereas your application sounds pretty important.

Modulating your return fan to a neutral floor pressure (or pressure in a return duct while dampers modulate to do the same) I've experienced to be very whimsical. Floor pressures are of the magnitude of ~0.0002 to 0.010 in. w.c. The difference in flow between two such arbitrary values can be enormous depending on how the whole place is put together: atria, openings, somebody keeping certain doors open you didn't anticipate, etc.

If you want robustness, here's what I would do:

Set all zones to full cooling (max VAV). Have the supply fan maintain duct static pressure - a set point that maintains the most remote box just under 100% open. Say this produces 9,000 cfm at 57 Hz. Adjust the return fan manually until you have 5,000 cfm, measured. Mark this VFD point. If this is 46 Hz, set return to equal 46 Hz when supply equals 57 Hz.

Set all zones to full heating (min VAV). Have the supply fan maintain the same duct static pressure. Suppose this is 5,000 cfm at 40 Hz. Adjust the return fan manually until you have 1,000 cfm, measured. Mark the VFD point. If this is 18 Hz, set return to equal 18 Hz when supply equals 40 Hz.

The sequence is simple; return tracks supply speed and requires a two-point calibration. Adjust supply minimums up as necessary to keep the minimum return fan speed above 15 Hz (note that this method should have volume dampering at each floor's returns, but no automatic flow control). This is not the only method possible, but one that could provide you the least headache.

 

[cry22]

First off - I'd say that your equipment choice is lousy (sorry for the term) for such a system, RTU with power exhaust for a hospital with isolation rooms is not the right equipment, you need a custum AHU using SF and RF plus Air flow measuring stations on SA, OA and RA to maintain you building balancing, the power exhaust fan will get your balancing into trouble. Plus, RTU's cannot give you the 95% filtration efficiency needed, they are not designed to use cartridge type filters.
You have a fan on VFD (SF) and power exhasut on Constant volume? incompatible operations.
Rf with VFD will track SF as mentionned to you earlier of course.

Installing dampers in RA ducts to be controlled by static is also a bad idea, just what is the static pressure on RA duct supposed to tell these dampers? how do these dampers know of airflow? how does your power exhaust react to these closed dampers? when the dampers close, your constant volume exhaust at the RTU will simply suck the main RA duct in and/or draw more OA than needed, OA for which your coil was not sized for.

Also, I'd use CHW if available nearby, because you are at close to 50% OA in humid Georgia (from what I understand) and a DX coil will not get you the proper dehumidification with that kind of OA. Use a CHW coil with minimum of 8-row coil.